I/O module for a serial multiplex data system with a programmable input voltage level selector

ABSTRACT

A programmable data link module is used on a time division multiplex data bus. The module receives data from the data bus during a selected time slot for a selected number of frames. The data link module includes an input circuit for generating a data output signal data to the data bus in response to input signals from an input device coupled to the module. The input circuit includes a programmable input voltage level selector, allowing different voltage rated input devices to be used with the data link module. A hysteresis circuit sets a minimum voltage level threshold representative of the input device being in one logic state and a maximum voltage level threshold representative of the input device being in another logic state.

This application is a division of application Ser. No. 08/305,253, filedSep. 13, 1994, U.S. Pat. No. 5,553,070.

FIELD OF THE INVENTION

This invention relates generally to the field of control systems and,more particularly, to control systems using data link modulescommunicating on a serial time division multiplex bus.

DESCRIPTION OF THE RELATED ART

Control systems employing a serial multiplex bus for controlling atleast one output device by a plurality of input devices are well known.Some known control systems use software protocols, operating under thedirection of a computer, in which all control signal data is conveyed inmulti-bit bytes or in packets of multi-bit bytes. Examples of suchsoftware protocol control systems are the LonWorks local operatingnetwork by Echelon Company of Palo Alto, Calif.; the home automationsystem Consumer Electronics Bus (CEbus) by the Electronic IndustryAssociation, the Controller Area Network (CAN) by Robert Bosch, GmbH ofStuttgart, Germany and the World Factory Implementation Protocol by theWorldFIP Committee of Research Triangle Park, N.C. Known softwareprotocol control systems disadvantageously require multi-bit bytes,typically 16-bit bytes, for conveying only one bit of data. Althoughsoftware protocol control systems are designed to convey multi-bit bytesor words, they are disadvantageously 10-100 times slower than hardwareprotocol systems, such as the invention, in conveying multibit words. Inmost software protocol control systems, the control protocol is composedof a header, the number of words in the transmission, loadidentification, load status and checksum. In most such systems, aminimum of six 8-bit words are needed to turn on one load. In somesoftware protocol systems, up to three times as many bits are required.

Software protocol systems have their communication capabilitiesdisadvantageously centralized, or lumped, in a computer that usessoftware to operate the control system; they disadvantageously requirethe computer in order to function, and therefore, if the computermalfunctions, then the control system will malfunction.

Other serial multiplex control systems use single bits of data to conveycontrol signals and such systems have their communication capabilitiesdistributed throughout the system, usually at each input and outputlocation. Most of these distributed, single-bit systems have hardwareprotocols without any programmability. An example of this type ofsingle-bit, hardware protocol control system is the Actuator SensorInterface (ASI) by ASI Verein eV Geschaftsfuhrung of Odenthal, Germany.Other examples are shown and described in U.S. Pat. Nos. 4,052,566 and4,052,567 issued Oct. 4, 1977 to MacKay; U.S. Pat. No. 4,156,112 issuedMay 22, 1979 to Moreland; U.S. Pat. No. 4,435,706 issued Mar. 6, 1984 toCallan; and U.S. Pat. No. 4,682,168 issued Jul. 21, 1987 to Chang, etal.

Hardware protocol systems are known to use a programmable logiccontroller (PLC) which is a computer programmed in ladder logic. Suchsystems disadvantageously require multiple lengthy cable runsinterconnecting the input and output devices to a terminal. Theexecution speed of a PLC computer is often too slow to provide real timeoperation.

Most known single-bit, hardware protocol systems are not programmable;however, an example of one such system that is programmable, throughfirmware, is described in U.S. Pat. No. 4,808,994 issued Feb. 28, 1989to Riley. Known programmable systems, such as that of the aforesaidpatent of Riley require additional dedicated terminals on the module foracceptance of programming information.

Known data link module integrated circuits have a transistor internal tothe integrated circuit for driving the data bus voltage low in order torepresent a signal in negative logic. Known data bus currents are aboutthirty milliamperes, and known data bus voltages are about twelve volts.However, the internal transistor used for driving the data bus low inknown data link module integrated circuits often fails when the data buscurrent and voltage becomes slightly higher than normal, such as fiftymilliamperes and sixteen volts.

Control systems are used in environments such as manufacturing andassembly factories and are exposed to electromagnetic noise, static, andspikes, pulses and transient voltages (herein referred to collectivelyas "noise"). Known data link modules passively rely upon the lack oftemporal coincidence between noise and signals to avoid noiseinterference. The presence of noise can cause an output device torespond at an inappropriate moment or fail to respond when the outputdevice should do so. Solely relying upon data signals being synchronizedwith an edge of a clock pulse has been found insufficient tosufficiently eliminate the effect of noise on a control system.

Known data link modules have a relatively narrow operating voltagerange, usually nine to thirteen volts, and it is impossible to use knowndata link modules with both twelve volt and the popular 24 volt systemswithout adding additional circuitry external to the integrated circuitfor conversion of voltages.

SUMMARY OF THE INVENTION

It is therefore the principal object of the present invention to providea data link module for use in a time division multiplexing controlsystem which overcomes the various disadvantages of the known data linkmodule.

This object is achieved in part by provision of a data link module withmeans for producing output control signals in response to input signalsreceived on a data bus input terminal on a time division basis during anassociated one of a plurality of time division multiplexing time slots,the improvement being an input signal conditioner with means forinitiating generation of an intermediate data pulse in response to thesignals at the data bus input terminal exceeding a pulse initiationthreshold voltage, means for terminating generation of the intermediatedata pulse in response to the signals at the data bus input decreasingbeneath a pulse termination threshold voltage different from the pulseinitiation threshold voltage, an intermediate pulse continuity checkerfor determining whether the intermediate data pulse is extant at each ofa plurality of occurrences of the address time slot associated with thedata link module and means responsive to the pulse continuity checkerfor producing a conditioned input signal only when it is determined thatthe intermediate data pulse is extant during each of said plurality ofaddress time slots.

The object is also achieved by providing a data link module powered by aDC supply voltage with means for producing an output control signal inresponse to data received on a data bus on a time division multiplexingbasis for use in a time division multiplexing control system having amaster clock for producing a master clock signal, the improvement beinga power on reset delay having means for detecting the application of DCsupply voltage to the data link module and means for inhibiting thecontrol signal producing means from changing the output control signalin response to data prior to the continuous application of the DC supplyvoltage for a preselected time period.

Also achieving the object is provision of a data ink module with anintegrated circuit with an data bus terminal for receipt of digital datasignals from a data bus, a local input terminal for receipt of inputsignals from a local input device, and a time division multiplexingaddress defining one of a plurality of time slots betweensynchronization pulses, a high voltage protection circuit having aswitchable breakdown device with an input junction and a pair oftransconductive outputs and having a breakdown voltage at which a shortis created between the transconductive outputs, means for connecting oneof the transconductive outputs to a data bus susceptible to voltagespikes in excess of the breakdown voltage and detrimental to theintegrated circuit if applied to the input, means for connecting theother transconductive output to a reference potential and meansresponsive to the address and to the signals at at least one of thelocal input terminal and the data bus terminal to provide a drive signalto the input junction of the switchable breakdown device to provide anappropriate data signal on the data bus during the time slot of theaddress of the data link module, said switchable breakdown devicebreaking down to protect the integrated circuit from said voltage spikeswhen not being driven to provide data signals to the data bus.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and advantageous features of the invention will beexplained in greater detail and others will be made apparent from thedetailed description of the preferred embodiment of the presentinvention which is given with reference to the several figures of thedrawing, in which:

FIG. 1 is a simplified schematic diagram of a control system usingpreferred embodiments of data link modules of the present invention;

FIGS. 2A and 2B form a composite functional block diagram of the circuitof a preferred embodiment of a data link module integrated circuit whichform part of the Output Modules and Input Modules, or data link modules,of FIG. 1;

FIG. 3A is a logic circuit diagram of the signal conditioning circuitassociated with Input A of the data link module of FIGS. 2A and 2B;

FIG. 3B is a more detailed circuit diagram of the programmablehysteresis circuit of the signal conditioning circuit of FIG. 3A;

FIG. 4 is a logic circuit diagram of the Power On Reset Delay functionalblock of FIGS. 2A and 2B;

FIG. 5 is a logic circuit diagram of the Input Inhibit, the Channel AInput Data Control, the Channel B Input Data Control and part of theWindow Control functional blocks of FIGS. 2A and 2B;

FIG. 6 is a logic circuit diagram of the CLOCK LOSS DETECT functionalblock of FIGS. 2A and 2B;

FIG. 7 is a logic circuit diagram of the Output Inhibit functional blockof FIGS. 2A and 2B;

FIG. 8 is a logic circuit diagram of the selectable Data Verifierfunctional block of FIGS. 2A and 2B;

FIG. 9 is a logic circuit diagram of the Polarity Independent functionalblock diagram of FIGS. 2A and 2B;

FIG. 10 is a logic circuit diagram of the Mode/Sync Output functionalblock of FIGS. 2A and 2B;

FIG. 11 is a logic circuit diagram of the multiplex address clock, orMUX CLOCK, functional block of the data link module of FIGS. 2A and 2B;

FIG. 12 is a logic circuit diagram of the Program Control functionalblock of FIGS. 2A and 2B;

FIG. 13 is a logic circuit diagram of the WORD EXTENDER, the ModeControl and part of the Window Control functional blocks of FIG. 1 toillustrate shift clock in and shift clock out aspects of the presentinvention;

FIG. 14 is a simplified circuit diagram of a data link module showing alogic circuit diagram of the Data Bus Drive functional block of FIGS. 2Aand 2B and a transistor driven by the Data Bus Drive output of FIGS. 2Aand 2B;

FIG. 15 is a simplified block diagram of a data link module of FIG. 1used as an output module, showing multiplexing;

FIG. 16 is a simplified block diagram of a data link module showing aData Bus Integrity Checker, the data link module integrated circuit andtwo 16-bit shift registers for 16-bit word addressing;

FIGS. 17A-17B and 17C-17D form composite timing diagrams showing themaster clock signal in relationship with various other signals;

FIG. 17E is an enlarged portion of FIG. 17D;

FIGS. 18A-18C and FIGS. 18D-18F form composite diagrams respectivelyshowing the program and verify cycles of the programming circuit of theinvention;

FIG. 19 is a logic circuit diagram of the Data Bus Integrity Checkerfunctional block of FIG. 16;

FIG. 20A-20B are timing diagrams of various signals involved in theoperation of the Data Bus Integrity Checker of FIG. 19; and

FIG. 21 is a set of timing diagrams of three frames of the clock bussignals and the data bus signals shown in detail in FIGS. 17A-17E.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a control system 30 using a plurality of data link modules32 constructed in accordance with the present invention. The controlsystem 30 is a hardware protocol system, programmable through firmware,that is capable of concurrently conveying single and multi-bit words ofdata. The control system 30 has its communication capability distributedamong a plurality of the data link modules 32. As a result, the use of acomputer 34 to operate the control system 30 itself is optional. Thecontrol system 30 also includes a master clock module 36 and a powersupply 38 which are both connected to a cable 40 preferably having fourconductors. The conductors include a direct current voltage power line42, a master clock line 44, a data bus 46 and a common 48 for the powersupply 38. The control system cable 40 is configured in any shaperequired such as the configurations known as ring, multidrop, loop back,bus and star. Of course, using ring or loop back configurations providesa degree of redundancy. One or more controlling or input devices 50,such as a photosensor controlled switch, are connected to at least oneof the data link modules 32. A data link module 32 having an inputdevice 50 connected to it acts as an input module for placing signals onthe data bus 46 on a serial multiplex time division basis in response tolocal input signals from the input device. One or more controlled, oroutput devices 54, such as a solenoid controlled switch or the like, areconnected to at least one of the modules 32. A data link module 32having an output device 54 connected to its local output acts as anoutput module for extracting data from the data bus 46 on the serialmultiplex time division basis and for producing local output signals towhich the output device responds. Each data link module 32 has up to twochannels for either inputting signals to the data bus 46 or outputtingsignals from the bus during a mode one time slot 65 or a mode two timeslot 67 shown in FIGS. 17A-17E. FIG. 1 shows, for simplicity ofillustration, each data link module 32 acting as either an input moduleor an output module, as shown in FIG. 1. Alternatively, a data linkmodule 32 can act as both an input and an output module concurrently. Insuch a case, an input device 50 connected to the data link module 32 isassociated with one of the channels and an output device 54 connected tothe data link module 32 is associated with the other of the channels.

The data link module operates in either a mode one or a mode two mode ofoperation. A timing diagram for mode one operation is shown in FIGS. 17Aand 17B. The control system 30, FIG. 1, uses a clock signal timingprotocol composed of an elongated sync period 58 followed by up to 256identical clock cycles 61 labelled 0-256. The sync period 58 and the 256clock cycles 61 represent a frame 62. The master clock signal 85 isgenerated by the master clock module 36 and is received by each datalink module 32 via the clock line 44 of bus 40. Each of the up to 256clock cycles 61 represents a potential device address. Each data linkmodule 32 selectively connects one or more of the input devices 50 andone or more of the output devices 54 to the data bus 46 during the timeslot, or address, 65 associated with each such device 50 and 54.Depending upon the mode of operation being mode one or of the a, theduration of the address 65 is one or two cycles, respectively, of thecontrol system master clock signal 85. The number of clock cycles 61 perframe 62 is selectable at the master clock module 36 and the numberselected is equal to the number of different addresses required. Thefewest number of clock cycles 61 needed per frame 62 are selected tooptimize the response time of the control system 30.

Referring to FIGS. 2A and 2B, each data link module 32 includes anintegrated circuit 80. As shown in FIGS. 17A and 17B, each mode one timeslot or address 65 is one full master clock cycle 61 in duration. Inmode one, during the first part of the time slot 65, data is placed onthe data bus 46 by one or more input data link modules 32 and the dataremains on the data bus 46 for the entire time slot. At the midpoint 64of the time slot 65, the data on the data bus 46 is copied from the busto output terminals 98 and 100 on at least one output data link module32 for use by at least one output device 54. New data is placed on thedata bus 46 at the end of the time slot 65, i.e. at the beginning of thenext time slot, and the process continues for each time slot 65 of aframe 62. Without multiplexing of frames 62, the process repeats itselfduring subsequent frames. The operation of the process with multiplexingof frames is described hereinafter.

The operation of the control system 30 using single bit input and outputsignals is described fully in the aforesaid patent of Riley which ishereby incorporated by reference. However, unlike known data linkmodules designed for the passing of single bits of data, the data linkmodule 32 of the present invention selectively passes either single bitor multi bits of data. For example, a control system 30 using the datalink modules 32 is thereby enabled to transfer a sixteen bit word ofdata from an input device 50 to the data bus 46 and from the data bus 46to an output device 54 without using complicated circuitry external tothe integrated circuit 80 of the data link module 32. Unlike known datalink modules, the data link module 32 is programmable to permit any sizeword of data up to 256 bits without any hardware change to the data linkmodule 32. In FIG. 1, the input and output devices are represented aseight bit devices merely for simplicity of illustration.

This multibit word feature allows the data link module 32 to easilyinteract with computer based input and output devices, but withoutrequiring a host computer 34 to operate the control system 32 itself.The variable length words allow the data link module 32 to interconnectpopular 8, 16 and 32 bit devices by making firmwire changes but withoutmaking major hardware changes. Analog data can be conveyed via the databus 46 of the control system 30 by using multibit words and by using ananalog to digital converter (not shown) at an input data link module 32and a digital to analog converter (not shown) at an output data linkmodule 32.

In addition, a control system using the data link module 32 overcomes alimitation of known devices since it is capable of controlling more than256 input devices and more than 256 output devices by multiplexingframes 62 without complicated circuitry external to the integratedcircuit 80 of the data link module 32.

The control system 30 using data link modules 32 constructed inaccordance with the invention selectively operates in mode one or modetwo. In mode two, the four conductors 42, 44, 46 and 48 of the controlsystem cable 40 are connected to an option host computer 34, preferablya microprocessor based personal computer, via a single half-slotindustry standard architecture (ISA) computer interface card (notshown). The control system 30 appears to the host computer 34 as a 2048byte block of dual-port random access memory (RAM). The input and outputis bit-mapped to an unused location of RAM.

Referring now to the mode two composite timing diagram, FIGS. 17C and17D, each mode two address 67 is twice as long (i.e. has twice theduration) as each mode one address 65, FIG. 17A. It should be noted thatthe scale of FIGS. 17A and 17B is not the same as the scale of FIGS. 17Cand 17D. Although the mode two master clock cycles 61 appear shorter inFIGS. 17C and 17D than they do in FIGS. 17A and 17B, this is only forconvenience of illustration; the mode two master clock cycles are thesame duration as the mode one master clock cycles 61. In mode two, anaddress 67, such as address "36" in FIG. 17E, is defined by two masterclock cycles 61 and 61', FIG. 17E. At the beginning of the first 61 ofthe two successive clock cycles 61 and 61' in the address 67, data isplaced on the data bus 46 by data link modules 32 acting as inputmodules. The data is latched on the data bus 46 for the duration of thefirst 61 of the two clock cycles 61 and 61'. The data is copied from thedata bus 46 by the optional host computer 34 during the first clockcycle 61. At the beginning of the second clock cycle 61' of the address67, the optional host computer 34 places a signal on the data bus 46,and the signal is latched for the duration of the second clock cycle61'. During the second clock cycle 61', data is copied from the data bus46 by a data link module 32 acting as an output module, and the data isfed to the output device 54 connected to the data link (output) module32.

Unlike known data link modules, the data link module 32 can operate witha power supply 38 of either approximately twelve volts or approximately24 volts without requiring adapters. And unlike known data link modules,the data link module 32 can be directly connected to either inputdevices 50 that represent binary logic levels with 0-5 volts or 0-9volts.

As shown in block form in FIGS. 2A and 2B, the integrated circuit 80includes input terminals 84, 86, 88, 90, 92, 94 and 96 for receipt of amaster clock signal, or CLOCK; data bus signals, or DATA; an operatingvoltage, or V_(cc) ; a voltage common, or COMMON, preferably at groundpotential; an external oscillator resistor, (not shown); a channel Ainput signal, or CH₋₋ A; and a channel B input signal, or CH₋₋ B,respectively. The integrated circuit 80 has output terminals 98, 100,102, 104, 106, 108, 110, and 112 for a channel A output signal, orOUTPUT₋₋ A; a channel B output signal, or OUTPUT₋₋ B; a channel C outputsignal, or OUTPUT₋₋ C; a shift clock in signal, or SH₋₋ CLK₋₋ IN; ashift clock out signal, or SH₋₋ CLK₋₋ OUT; a multiplex clock signal, orMUX; a mode/synchronizing signal, or MODE₋₋ SYNC; and a data drivesignal, or DATA₋₋ DRV, respectively.

Each data link module 32 has two channels, channel A and channel B, andeach channel is associated with an address. During each time frame 62,an eight bit counter 114 shown in FIG. 2A counts the cycles 61 of themaster clock signal 85. The eight bit counter 114 is reset at thebeginning of each frame 62. In mode one, each clock cycle 61 correspondsto an address 65. In mode one, the counter 114 counts every clock cycle61 and the results are fed to address comparators 116 and 118 thatcompare the count of the counter with the respective addresses for whichthe data link module 32 has been programmed. When the count matches theaddress, the associated comparator generates a coincidence A (COIN₋₋ A)and a coincidence B (COIN₋₋ B) signal, respectively. In mode two, theoperation is similar except an address 67 consists of two clock cycles61 and 61'. As shown in FIG. 2A, the window control section 120 of theintegrated circuit 80 feeds a signal to the eight bit counter 114 tocompensate for the difference between mode one and mode two.

The operation of the integrated circuit 80 will become clear after theoperation of the various sections of it, represented by the blocks inFIGS. 2A and 2B, are individually described in detail hereinafter.Except when essential to understanding the operation of the variouscircuits, the power supply line 42 and the common line 48, are not shownin the detailed diagrams of the circuits.

Signal Conditioning

As shown in block form in FIG. 2A, the channel A input signal at thechannel A input terminal 94 to the integrated circuit 80 is fed througha dual signal conditioning circuit 180 before further processing. Thesignal conditioning circuit 180 includes a Channel A signal conditioningcircuit 122 (shown in FIG. 3A) and a Channel B signal conditioningcircuit (not shown). As shown in FIG. 3A, the signal conditioningcircuit 122 has an anti-aliasing filter 124, a hysteresis circuit 126,and a digital low pass filter 128. The signal conditioning circuit (notshown) for channel B is essentially identical to the signal conditioningcircuit 122 for channel A; therefore, only the signal conditioningcircuit for channel A will be described in detail. The input terminal130 of the signal conditioning circuit is the input terminal 131 to theanti-aliasing filter. The anti-aliasing filter 124 includes a seriesresistor 23 preferably approximately 470 kΩ, and a capacitor 156 betweenthe input 131 and ground potential, preferably approximately 17 pF.Preferably, the invention of the data link module 32 is used withchannel A and channel B input frequencies below 3 kHz; therefore, theanti-aliasing filter 124 has a bandstop of approximately 30 kHz. Theoutput 132 of the anti-aliasing filter 124 is fed into an input 134 ofthe hysteresis circuit 126.

The hysteresis circuit 126 is programmable to accept a signal havingdifferent voltage levels ranges at its input terminal 134. A cell 138 ofan electrically erasable read only memory (EEPROM), not shown in itsentirety in FIG. 3A, is connected to another input 140 of the hysteresiscircuit 126. A logical zero at the cell 138 causes the acceptable inputvoltage range to be 0-5 volts. A logical one at the cell 138 causes theacceptable input voltage range to be 0-9 volts. The hysteresis circuit126 prevents false state transitions due to minor voltage levelvariations. The programmable hysteresis circuit 126 and the cell 138 ofthe EEPROM function together as a local input voltage range selector.

The hysteresis circuit 126, shown in more detail in FIG. 3B, includes aninverter 142 for inverting the value stored in the EE cell 138, and acircuit 144 of eight resistors and four transistors for producing tworelatively high and two relatively low voltages. The hysteresis circuit126 is preferably a 50% hysteresis circuit. The two relatively highvoltages, 6.75 v and 3.75 v, are 75% of the maximum input voltageexpected in 9 v systems and 5 v systems, respectively. The tworelatively low voltages, 1.25 v and 2.25 v, are 25% of the maximum inputvoltage expected in 9 v systems and 5 v systems, respectively.Activation and deactivation of the EE cell 138 causes selection of oneof the pair 6.75 v and 2.25 v and the pair 3.75 v and 1.25 v for use bycontrolling comparators 146 and 148. One of the comparators 148 producesa comparator output signal (HITRIP) 149 in response to a signal at theinput terminal 134 of the comparator 148 exceeding 75% of the maximuminput voltage. The input terminal 133 of the comparators 146 and 148 isconnected to the input terminal 134 of the hysteresis circuit 126. Thecomparator output signal (HITRIP) 149 sets a flip-flop 150. The Q outputterminal 152 of the flip-flop 150 is the output terminal of thehysteresis circuit 126. The output signal 135 of the hysteresis circuit126 is a flat wave that remains high until the voltage of the signal 125at the input terminal 134 of the hysteresis circuit falls below 25% ofthe maximum input voltage, at which time the other comparator 146produces another comparator output signal (LOTRIP) 147 to reset theflip-flop 150 thereby bringing the signal 135 at the output terminal 152of the hysteresis circuit to zero volts. The signal 135 at the outputterminal 152 of the hysteresis circuit 126 is feed into an inputterminal 154 of the digital filter 128. An output signal 135 from of aninternal oscillator 158 shown in block form in FIG. 2B is feed into thedigital filter 128 to control the bandstop of the digital filter. Thebandstop frequency is determined by the frequency of the internaloscillator 158 divided by five. Preferably, the frequency of theinternal oscillator 158 is approximately 150 kHz. The digital filter 128is comprised of a multi-bit, preferably five bit, serial-in,parallel-out shift register 160. The output signal 163 from the shiftregister 160 is simultaneously presented to a positive pulseconsecutivity checker 162 and to a negative pulse consecutivity checker164, each comprised of a multi-input, preferably five-input, AND gate166 and 168, respectively. The positive pulse consecutivity checker 162produces a signal at input terminal S 170 of an RS flip-flop 172 onlywhen a signal is extant during each of a plurality of consecutive cyclesof the internal oscillator 158. The RS flip-flop 172 retains at output Qterminal 174 a logical one output until reset by the negative pulseconsecutivity checker 164. The output Q terminal 174 of the RS flip-flop172 is connected to the output terminal 176 of the channel A signalconditioning circuit 122. An IINPUT₋₋ A signal, being a conditionedINPUT₋₋ A signal, is produced on the output terminal 176 of the channelA signal conditioning circuit 122. The output terminal 176 of the signalconditioning circuit 122 for channel A and the output terminal 178 ofthe signal conditioning circuit (not shown) for channel B are coupled tothe channel A and the channel B, respectively, input data controlcircuits 182 and 184.

The data bus 46 and master clock bus 44 conditioning circuits 186 and188, respectively, operate in a manner similar to the channel A and Binputs signal conditioning circuit 122 (not shown) except for thefrequencies involved. Preferably the invention of the data link module32 is used at clock bus and data bus 46 frequencies below 200 kHz;therefore, the data bus and clock bus anti-aliasing filters 186 and 188have an anti-aliasing filter resistor 126 of preferably 100 kΩ and thesignal conditioning circuits preferably have a break point ofapproximately 225 kHz. In a similar manner, the data bus signalconditioning circuit 186 and the master clock bus signal conditioningcircuit 188 produce conditioned signals IDATA 191 and ICLOCK 192 fromsignals DATA 87 and CLOCK 85, respectively.

Power On Reset Delay

Referring now to FIG. 2B, the internal oscillator 158 of the integratedcircuit 80 has a frequency range of preferably 50-400 kHz. The frequencyis controlled by the external resistor R (not shown) connected frominput terminal 92 to ground, where R (in ohms)=14.5×10⁹ /freq. (inHertz). Preferably, R is between 25-200 kΩ. The internal oscillator 158is of a type generally known and is not part of the invention. Theinternal oscillator 158 produces a signal OSC 159.

As shown in block form in FIG. 2B, a power on reset delay circuit 190has three input terminals 181, 183 and 185 for receipt of the oscillatorsignal (OSC) 159, the internal master clock signal (ICLOCK) 192, and apower on reset signal (POR) 194. The POR signal 194 occurs immediatelyafter application of power to the integrated circuit 80 and is producedby one of several well known power-on-reset means 195 and is not part ofthe invention. The power on reset delay circuit 190 has two outputterminals 216 and 218 for outputting a synchronous signal (SYNC) 196 anda power on reset delay signal (POR₋₋ DLY) 198, respectively.

The power on reset delay circuit 190 is shown in detail in FIG. 4. Theinternal master clock signal (ICLOCK) 192 is held high for a preselectedsync period 58 in order to define the beginning of each time frame 62.Preferably, the sync period 58 has a duration of eight master clockcycles 61. A four bit counter 200 and a NAND gate 202 produce a lowSYNC₋₋ DET signal 205 at the output terminal 204 of the NAND gate 202upon receiving ten cycles of the OSC signal 159 while ICLOCK 192 is,during the same interval, continuously high. The ICLOCK signal 192 isfed into an inverted clear input 183 of the counter 200. The SYNC₋₋ DETsignal 205 remains low for a period dependent upon the relationshipbetween the ICLOCK frequency and the OSC frequency; however, on thefirst negative transition of ICLOCK, which defines the end of the syncperiod 58, SYNC₋₋ DET become high again because the negative ICLOCKpulse clears the counter 200. Preferably, the OSC frequency is one andone-half to ten times faster than the ICLOCK frequency.

The SYN₋₋ DET signal 205 is fed into a plurality of interconnected dataflip-flops 206 and 208 and then into an AND gate 210. The flip-flops 206and 208 are cleared upon receipt of the low POR signal 194 at flip-flopinputs 209 and 211, respectively. The output 212 of the AND gate 210produces the low POR₋₋ DLY signal 198 immediately after receipt of thelow POR signal 194. The output 212 of the AND gate 210 produces a highPOR₋₋ DLY signal 198 after the plurality of data flip-flops 206 and 208receive a plurality of SYNC₋₋ DET signals via gate 201.

The circuit shown in FIG. 4 produces the high POR₋₋ DLY signal 198 afterreceiving three SYNC₋₋ DET signals 205. Although only two dataflip-flops 206 and 208 are shown in FIG. 4 for simplicity, it ispreferable to have approximately 12 data flip-flops for counting 4,095SYNC₋₋ DET signals 205 prior to producing the high POR₋₋ DLY signal 198.The power on reset delay circuit 190 produces the POR₋₋ DLY signal 198after the integrated circuit 80 receives a preselected plurality ofSYNC₋₋ DET signals 205 after application of power to the integratedcircuit. The POR₋₋ DLY signal 198 and the SYNC₋₋ DET 205 signal aregated through an AND gate 214 to produce a SYNC signal 196 at the output216. Therefore, SYNC 196 remains low unless POR₋₋ DLY 198 is high.

Many of the flip-flops of the integrated circuit 80 are either preset orcleared by SYNC 196; therefore, these flip-flops are advantageouslypreset or cleared only after the power supply to the data link module 32has stabilized. The POR₋₋ DLY signal 198 is fed to the clock loss outputlock circuit 240, to the safety input inhibit protection circuit 220,and to the mode/sync circuit 458.

Safety Input Protection Circuit

As shown in block form in FIGS. 2A and 2B, the safety input protectioncircuit or input inhibit circuit 220 prevents the input data controlcircuits 182 and 184 from responding to input signals during apreselected period of time after power is applied to the integratedcircuit 80 and also during the period of time that the integratedcircuit is being programmed or the programming is being verified. Asshown in more detail in FIG. 5, the input inhibit circuit 220 has an NORgate 224 having the active low POR₋₋ DLY signal 198 and an active highprogram/verify mode signal (P/V₋₋ MODE) 199 as inputs fed into inputterminals 226 and 228, respectively, and having an INPUT₋₋ INHIBITsignal 229 as an output signal at output terminal 230.

The low POR₋₋ DLY signal 198 is produced by the power on reset delaycircuit 190 for a preselected period of time after power is applied tothe integrated circuit 80. A high P/V₋₋ MODE signal 199 is produced bythe module programmer 232 during the programming cycle and during theverifying cycle. The POR₋₋ DLY signal 198 and the P/V₋₋ MODE signal 199are applied to the inputs 226 and 228 of the input inhibit circuit 220.The output terminal 230 of the input inhibit circuit 220 is coupled toinput terminals 416 and 418 of the channel A and channel B input datacontrol circuits, 182 and 184, respectively. The input inhibit circuit220 works in conjunction with the channel A and channel B input datacontrol circuits, 182 and 184, respectively. The operation of thechannel A input data control circuit is essentially the same as theoperation of the channel B input data control circuit so only thechannel A circuit will be described in detail. As shown in FIG. 5, theINPUT₋₋ INHIBIT signal 229 at the input terminal 416 is fed into aninverted clear (CLR) terminal 235 of a flip-flop 400. As a result, asignal at an output Q terminal 406 of the flip-flop 400 becomes zerowhen either the signal POR₋₋ DLY 198 is low or the signal P/V₋₋ MODE 199is high. Therefore, the subsequent stages of the integrated circuit 80will not respond to input A or input B signals under preselectedconditions to enhance safety and reliability of the data link module 32.Preferably, the preselected conditions are when either the signal POR₋₋DLY 198 is low or when the signal P/V₋₋ MODE 199 is high.

Clock Loss Output Lock

It is advantageous to prevent signals at the output terminals 98, 100,102, 104, 106 and 108 of the integrated circuit 80 from changing in theabsence of the master clock signal 85. The integrated circuit 80 has aclock loss detect circuit 240 shown in block form in FIG. 2B. The clockloss detect circuit 240 shown in more detail in FIG. 6 has three inputterminals 221-223 for receipt of ICLOCK 192, OSC 159 and POR₋₋ DLY 198signals and one output terminal 225 producing a long reset signal(LONG₋₋ RST) 241. The clock loss detect circuit 240 includes a thirteenbit counter 242 which starts to count the cycles of OSC 159 if there isa loss of the internal master clock signal ICLOCK 192. The OSC signal159 is fed to the clocking pin 244 of the thirteen bit counter 242. Theloss of ICLOCK 192 is determined by a delay element (DLY) 245 and anexclusive-OR gate 246 connected to the clear (CLR) pin 248 of thecounter 242. When ICLOCK 192 is extant, the counter 242 is cleared onevery transition of ICLOCK. Upon counting 6,144 cycles of the OSC signal159, outputs 250 and 252 of the counter 242, representing the twelfthand thirteen bits of the number 6,144 in binary, are high which causes aNAND gate 256 to produce a low clock loss delay (CLK₋₋ LOSS₋₋ DLY)signal at its output 254.

The clock loss detect circuit 240 begins to produce a low CLK₋₋ LOSS₋₋DLY signal commencing 6,144 cycles of OSC after loss of the master clocksignal and the circuit continues to produce the low CLK₋₋ LOSS₋₋ DLYsignal until reinstatement of the master clock signal. The POR₋₋ DLYsignal and the CLK₋₋ LOSS₋₋ DLY signal are coupled through an AND gate258 to produce a long reset (LONG₋₋ RST) signal. The LONG₋₋ RST signalis fed to the repeat circuit 260 and to the safety output protectioncircuit 262.

Safety Output Protection Circuit

It is important to prevent the multiplex clock 108, shift clock out 106,shift clock in 104, the A, B, and the C outputs 98, 100 and 102terminals from outputting under certain conditions. As shown in blockform in FIG. 2B, the integrated circuit 80 has a safety outputprotection circuit 262 that prevents the output terminals 98, 100, 102,104, 106 and 108 of the integrated circuit 80 from having an outputsignal in the presence of either a low LONG₋₋ RST signal or duringprogramming or verifying of the programming.

As shown in more detail in FIG. 7, the safety output protection circuit262 has three input terminals 264, 266 and 268 for receiving SYNC,LONG₋₋ RST and a program voltage enable (PVE) signal, respectively. Thecontrol system power line voltage V_(cc) is applied continuously to a Dinput terminal 270 of a data flip-flop 272. The data flip-flop 272produces a high output inhibit signal at Q 274 as a result of thevoltage V_(cc) applied to the D input terminal 270 of the flip-flop 272,except when cleared by one of a high PVE signal and a low LONG₋₋ RSTsignal. The circuit also includes an NOR gate 276 that combines the highPVE and the low LONG₋₋ RST signals to form one reset signal at the NORgate output terminal 277 prior to the signals being applied to a clearpin 278 of flip-flop 272. A low reset signal clears the flip-flop 272.The SYNC signal enables the flip-flop 272 at the beginning of each frame62 and as a result the flip-flop is set by the first SYNC signal afterthe reset signal is removed. Referring now to FIGS. 2A and 2B, theoutput of the safety output protection circuit 272 is an output inhibitsignal (OUTPUT INHIBIT) 280 which is fed to a tristate device at each ofthe output terminals 98, 100, 102, 104, 106 and 108, respectively,forcing those output terminals of the integrated circuit 80 into a highimpedance state. The high impedance state of the output terminals 98,100, 102, 104, 106, and 108 is indicative of an absence of both a highsignal and a low signal at the output terminals.

Data Verifier

Referring to FIG. 8, the repeat circuit, or data verifier, 260 shown inblock form in FIG. 2A has a channel A data verifier 288 and a channel Bdata verifier 289 which is substantially identical to the channel A dataverifier. In order to avoid erroneous responses to random noise whichmay be present on the data bus 46 during a selected time slot 65 and 67associated with one of the modules 32, the data verifier 260 selectivelyrequires a repetition of the same data signal 87 on the data bus 46during the time slot 65 and 67 associated with the data link module 32for a selected plurality of successively contiguous time frames 62, 62'and 62", FIG. 21. Only after said repetition will the appropriatechannel A or channel B output signal change in response to the datasignals 87 on the data bus 46. Still referring to FIG. 8, the channel Adata verifier 288 includes a plurality of binary data flip-flops 282,284 and 286 interconnected to form a multistage shift register. Each ofthe data flip-flops 282, 284 and 286 stores the signal 87 present on thedata bus 46 during each one of a plurality of successive time frames 62,62' and 62", FIG. 21. The input terminals 290, 292, 294, and 298, FIG.8, of the channel A data verifier receive the signals from the data bus46, from a pair of electrically erasable read only memories, or EEcells, 320 and 322 of an EEPROM 354, and from the clock loss circuit240, respectively. Input terminal 296 of the channel A data verifierreceives an enabling clock pulse 302 produced on the output 303 of anAND gate 304. The AND gate 304 has three inputs 311, 313, and 315 forreceiving ICLOCK, COIN₋₋ A and OUTPUT₋₋ WINDOW signals, respectively.The selected mode of operation, mode 1 or mode 2, determines the natureand timing of these signals.

In mode 1, there is one clock cycle 61 per time slot 65. The OUTPUT₋₋WINDOW signal is continuously low, and an inverter 312 at input 311permits the 3-input AND gate 304 to be continuously enabled. Anotherinverter 314 at input 315 inverts the ICLOCK signal 192 to enable thethree-input AND gate 310 during only the second half of the clock cycle61 when ICLOCK is low. The COIN₋₋ A signal is high only during theoccurrence of the selected time slot 65 associated with the data linkmodule 32. The edge sensitive data flip-flops 282, 284 and 286 areenabled when COIN₋₋ A transitions from low to high. The data flip-flops282, 284 and 286 are enabled during, and only during, the selected timeslot 65 associated with the data link module 32.

In mode 2, there are two clock cycles 67 and 67' per time slot 67. Inmode 2, during the first clock cycle 61, the OUTPUT₋₋ WINDOW signal ishigh and during the second clock 61' cycle the OUTPUT₋₋ WINDOW signal islow. Therefore, the three-input AND gate 304 is activated only duringthe second 61' of the two clock cycles 67 and 67'. In other respects,the data verifier 260 works the same in mode two as it does in mode one.

The enabling clock pulse 302 enables each of the three data flip-flops282, 284 and 286 during the time slot, one of 65 and 67, associated withchannel A of the data link module 32, but within successive frames 62,62' and 62", FIG. 21. Initially, the data flip-flops 282, 286 and 288are preset by a LONG₋₋ RESET signal from the circuit of FIG. 6.Thereafter, upon the first occurrence of the time slot 61 of channel Aof the data link module 32, the data signal on the data bus will appearat the Q output 324 of the first flip-flop 282 and also at the D input316 of a repeat circuit output flip-flop 318. The signal 87 on the databus 46 during the most recent frame, one of 62, 62' and 62", will alwaysbe reproduced at the D input 316 of the repeat circuit output flip-flop318, but the repeat circuit output flip-flop 318 will be enabled onlyupon the occurrence of the enabling clock pulse 302 from output 303 ofAND gate 304. The occurrence of the enabling clock pulse 302 at therepeat circuit output flip-flop 318 is controlled by programming.

When the EE cell 320 is programmed to contain a logical zero, an ANDgate 338 switches its output 336 to a logical zero, and an output 340 ofNAND gate 342 switched high during the second half of each clock cycle61 to enable the repeat circuit output flip-flop 318, without anyrepetition of data, at a time one-half of a clock cycle 61 after theactivation of the first flip-flop 282. When the EE cell 320 isprogrammed to contain a logical one and an EE cell 322 is programmed tocontain a logical zero, the clock of the output flip-flop 318 will beenabled only if both the first and second flip-flops 282 and 284 havethe same Q outputs. The first 282 and second 284 flip-flops will havethe same Q output only if the data signal 87 on the data bus 46 has beenrepeated in the selected time slot, one of 65 and 67, in each of twosuccessive frames 62 and 62'. The Q outputs 324 and 326 of the first andsecond flip-flops 282 and 284, respectively, are fed into anexclusive-OR gate 328.

The signal produced on the output 330 of the exclusive OR gate 328, incombination with the signal produced on the output of a NOR gate 344 anda NAND gate 346 and the state of the EE cells 320 and 322 will cause therepeat circuit output flip-flop 318 to be enabled immediately after(i.e. one-half clock cycle 61 after) a frame 62' having a secondconsecutive repetition of data signals 87.

When both EE cells 320 and 322 are programmed to contain a logical one,the clocking pin of the repeat circuit output flip-flop 318 is enabledimmediately after (one-half clock cycle 61 after) a third frame 62",only if the first, second and third flip-flops have the same signals ontheir Q outputs 324, 326 and 332, respectively. The signal on output 232of flip-flop 286 is fed into an exclusive-OR gate 348 along with thesignal on output 324 of flip-flop 282. The signal on output 349 of gate348 is low only when there is an identity of data between the first 62and third frames 62".

Table 1 summarizes the effect on the data verifier of programming EEcells 320 and 322.

                  TABLE 1    ______________________________________    EE CELL            320    322    ______________________________________    No Repetitions     0      0 or 1    One Repetition     1      0    Two Repetitions    1      1    ______________________________________

In a similar manner, if needed, the data verifier 260 is extended torecognize and selectively respond to more than a minimum of threeoccurrences of the same data signals 87 on consecutive frames 62 byadding more flip-flops and more EE cells and their associatedprogrammable logic circuitry.

Polarity Selector Circuit

In order to overcome the deficiencies of the C output terminal of knowndata link modules, the data link module 32 has a combinational logiccircuit polarity selector 350, shown in block form in FIG. 2B,comprising a polarity independent circuit 352, FIG. 9. The polarityindependent circuit 352 receives the A₋₋ OUTPUT signal and the B₋₋OUTPUT signal from the data verifier 260 and also receives polarityselection information from the EEPROM 354. As shown in detail in FIG. 9,the polarity independent circuit 352 includes an AND gate 356 having twoinput terminals 358 and 360 for receiving the A₋₋ OUTPUT and B₋₋ OUTPUTsignals and having an output terminal 362 for producing a C₋₋ OUTPUTsignal. Unlike known data link modules, the polarities of the inputsignals to the AND gate 356 of the invention are not restricted to beingthe same as the polarities of the A₋₋ OUTPUT and B₋₋ OUTPUT signals. Thepolarity of each input to the AND gate 356 can be individually,selectively changed from the polarities of the A₋₋ OUTPUT and B₋₋ OUTPUTsignals. Polarity selection information from one of the EE cells 364 andthe A₋₋ OUTPUT signal are fed into an exclusive-OR gate 366. The output368 of the exclusive-OR gate produces the A₋₋ OUTPUT signal but with apolarity selectively different than its input polarity. Similarly, theB-OUTPUT signal and a second EE cell 370 is fed through anotherexclusive-OR gate 372. The polarity of the output of the AND gate 356 isselectively controlled by a third EE cell 374 and an exclusive-OR gate375 thereby allowing C₋₋ OUTPUT signal to be any logical combinationalfunction of the A₋₋ OUTPUT signal and B₋₋ OUTPUT signal.

Input Synchronizer

Referring now to FIG. 2A, the integrated circuit 80 has input datacontrol circuits 182 and 184 to isolate subsequent portions of theintegrated circuit from changes in channel A and channel B input signalsoccurring during a time slot 65 associated with a data link module 32.The channel A input data control circuit 182 is substantially identicalto the channel B input data control circuit 184, therefore only thechannel A input data control circuit will be described in detail. Thechannel A input data control circuit 182 has COIN₋₋ A, IINPUT₋₋ A,INPUT₋₋ WINDOW, and INPUT₋₋ INHIBIT signals as inputs and a INPUT₋₋DATA₋₋ A signal as an output. As shown in more detail in FIG. 5, theinput data control circuit 182 has a data flip-flop 400 and athree-input AND gate 402. The IINPUT₋₋ A signal is fed into the D input404 of the data flip-flop 400 which is enabled by the COIN₋₋ A signalonly at the beginning of the time slot 65 associated with the data linkmodule 32. The COIN₋₋ A signal is normally low and goes high only duringthe address associated with the data link module 32. The data flip-flop400 is edge sensitive. At the rising edge of the COIN₋₋ A signal thestate of the IINPUT₋₋ A signal is latched onto a Q output 406 of theflip-flop 400 for the duration of the time slot 65 of the data linkmodule 32. The Q output 406 is fed into the three-input AND gate 402along with COIN₋₋ A and INPUT₋₋ WINDOW. In mode one, INPUT₋₋ WINDOW iscontinuously low and the inverter 408 at one of the inputs 410 of theAND gate 402 allows the AND gate to be enabled by INPUT₋₋ WINDOW. Duringthe time slot 65 of the data link module 32, COIN₋₋ A is high and theAND gate 402 is enabled by COIN₋₋ A during the module's time slot.Therefore, in mode one, the IINPUT₋₋ A signal appears at the outputterminal 412 of the AND gate 402 only during the time slot 65 of thedata link module 32. Referring now to the timing diagram, FIGS. 17C and17D, in mode two, each time slot 67 is twice as long as each time slot65 in mode one, but only the first half of each mode two time slot 67 isdedicated to applying input signals to the data bus 46. In mode two,INPUT₋₋ WINDOW operates at half the frequency of the master clock signal85. In mode two, INPUT₋₋ WINDOW is low during the first half of the timeslot 67 and high during the second half of the time slot. Referringagain to FIG. 5, the inverter 408 on the input 410 to the AND gate 402allows the AND gate to be enabled only during the first half of the timeslot 67 of the data link module 32. Therefore, in mode 2, the IINPUT₋₋ Asignal appears at the output 412 of the AND gate 402 only during thefirst half of the time slot 67 of the data link module 32. The output412 of the AND gate 402 forms the output terminal 414 of the input datacontrol circuit 182 at which is produced the signal INPUT₋₋ DATA₋₋ A.The INPUT₋₋ DATA₋₋ A signal is fed to the data bus drive circuit 420.

Mode Selection Indicator

Referring now to FIG. 15, in order to synchronize the operation of othercomponents, such as shift registers 588 and 590, on the data link module32 with the operation of the integrated circuit 80 and in order toinform the other components on the module of the mode of operation, theintegrated circuit has a single terminal 110 for alternately conveying aSYNC signal 196 and a MODE signal 488. As shown in block form in FIG.2B, the combined MODE/SYNC signal 456 is generated by a mode/sync outputcircuit 458. The mode/sync output circuit 458 has four input terminalsfor receipt of MODE 488, SYNC 196, OSC 136, and POR₋₋ DLY 198 as inputsignals and one output terminal 468 for producing the combined MODE/SYNCas an output signal 456. Advantageously, the MODE/SYNC signal 456 isindependent of the channel A and channel B input signals, and of thedata signals 87 on the data bus 46. The MODE/SYNC output terminal 468periodically produces a SYNC output signal 196 whenever the integratedcircuit 80 has operating power, except for a short period of timeimmediately after start up due to a low POR₋₋ DLY signal. Of course,there can be no sync information on the MODE/SYNC output terminal 468 ifthe integrated circuit 80 loses the master clock signal 85, but the modeinformation will remain on the MODE/SYNC output terminal 468 in such anevent.

The mode/sync output circuit 458 is shown in more detail in FIG. 10. Thelow POR₋₋ DLY clears a data flip-flop 476. An inverted SYNC signal 196and a delayed SYNC signal 196 are fed into an AND gate 474 to produce apositive short duration pulse on a transition when SYNC falls low. Theshort duration pulse presets the data flip-flop 476. A D input 478 ofthe flip-flop 476 is continuously at ground potential. The OSC signal136 is fed into the clocking input 480 of the flip-flop 476. Therefore,the Q output 482 of the flip-flop 476 is normally low but it is presethigh by the short duration pulse at the beginning of every frame.However, the output Q 482 of the flip-flop 476 remains high for only onecycle of OSC 136 and then it returns low. The Q output 482 of theflip-flop 476 goes into an exclusive-OR gate 484 along with the MODEsignal 488. The MODE signal reflects the mode selected and stored in theEEPROM 354 during programming. In mode one, MODE=0 and the exclusive-ORgate 484 effectively inverts the Q output 482 of the flip-flop 476. Inmode two, MODE =1 and the exclusive-OR gate 484 does not change the Qoutput 482 of the flip-flop 476. An inverter/buffer 486 inverts theMODE/SYNC signal 456', regardless of mode, prior to the MODE/SYNC signal 456appearing on the MODE/SYNC output terminal 110 of the integrated circuit80. As shown on the timing diagram, FIGS. 17A and 17B, in mode one,MODE/SYNC 456 is continuously low, except that it goes high for onecycle of OSC 136 at the beginning of each frame 62. In mode two,MODE/SYNC 456 is continuously high, except that it goes low for onecycle of OSC 136 at the beginning of each frame 62.

Multiplexing Frame Identifier

Referring now to FIG. 21, multiplexing of frames 62 allows a successiveframe 62' to convey different data than the data conveyed by a precedingframe 62. Referring now to FIG. 2A, the integrated circuit 80 has amultiplex clock circuit 490 for generating a multiplex clock signal 492needed to multiplex frames 62. The integrated circuit 80 also providesan output terminal 108 so that the multiplex clock signal (MUX₋₋ CLK)492 could be accessed by relatively few and relatively simple externalcomponents on the data link module 32. This feature overcomes one of thedisadvantages of the prior out, which required similar signals to begenerated by additional external and delicate components.

Referring now to FIG. 16, integrated circuit 80 has a multiplex addressclock output terminal 108 for use by a decoder 494 on the data linkmodule 32 for selectively enabling a plurality of shift registers 496,498 and 500. Each of the plurality of shift registers 496, 498 and 500transfers data from the data bus 46 to an associated output field device54, 54', 54" during the same time slots 65 but within different frames62. The use of the MUX₋₋ CLK signal 492 permits time divisionmultiplexing of frames 62. When time division multiplexing is done, eachframe 62 is given a frame number. Time slots 1-4, inclusive, are used tonumber each frame 62. Time slot 0 is not used in frame multiplexing. Themaster clock module 36, FIG. 1, places the frame number on each frame 62by putting a series of four signals, representative of the frame number,on the data bus during time slots 1-4 of each frame. Because four bitsare used to assign frame numbers, up to sixteen different numbers can beassigned to frames. The multiplexing of 16 frames allows for the serialtransmission of up to 3,840 bits of data (16×240 bits per frame).Although the present embodiment uses four time slots for multiplexing upto 15 frames, it is possible to use up to sixteen time slots formultiplexing up to 32,768 frames to convey up to 7,864,320 bits of data(32,768×240 bits per frame).

As shown in block form in FIG. 2A, the multiplex clock circuit 490 haseight parallel input terminals 501-508 for acceptance of eight-bit frameaddresses, and terminals 509, 510 and 511 for acceptance of SYNC, ICLOCKand OUTPUT₋₋ WINDOW signals, respectively. The circuit has one outputterminal 513 for outputting a multiplex clock (MUX₋₋ CLK) signal 492.

As shown in more detail in FIG. 11, the multiplex clock circuit 490includes an eight-input 501-508 NAND gate 514 with six inputs havinginverters for detecting a frame 62 having a frame number 00000101=5. TheNAND gate 514 produces a COUNT₋₋ 5 signal which is normally high butwhich is low for one master clock cycle 61 after detecting the framewith the frame number five.

The SYNC signal 196 sets an RS flip-flop 512 at the beginning of eachframe 62 which produces a high output at Q 515 which, in turn, enables athree-input AND gate 516. In mode one, the OUTPUT₋₋ WINDOW signal 121 iscontinuously low and also enables the three-input AND gate 516. Thecombination of the SYNC 196 and OUTPUT₋₋ WINDOW 121 signals enablesICLOCK 192 to be reproduced at the output terminal 513 of the multiplexclock circuit 490 until the fifth frame is counted by counter 114. Whenthe fifth frame is counted a COUNT₋₋ 5 signal is produced by NAND gate514, and flip-flop 512 is reset. The output Q of flip-flop 512 becomeszero thereby disenabling the three-input AND gate 516 after counter 114counts five frames 62. After the fifth frame 62, no MUX₋₋ CLK signal 492is produced.

In mode two, the multiplex clock circuit 490 works similar except thatOUTPUT₋₋ WINDOW 121 is a clocking signal operating at half the frequencyof ICLOCK 192 and in phase with ICLOCK. In mode two, the three-input ANDgate 516 enables ICLOCK 192 to pass through to the multiplex clockoutput terminal 513 only when both OUTPUT₋₋ WINDOW and ICLOCK are bothnegative. As shown on the timing diagram, FIG. 17C, in mode two, MUX₋₋CLK 492' is a train of positive and negative pulses of dissimilar width.

As shown in FIG. 16, the MUX₋₋ CLK signal 492 is fed into an input 518of the decoder 494. The data bus is connected to another input 519 ofthe decoder 494. The MUX₋₋ CLK signal 492 allows the decoder 494 toserially receive the frame numbers from the data bus 46 during timeslots 1-4. The output of the decoder 494 has up to 16 individual linesfor successively enabling one of each of the shift registers 496, 498and 500 during the frame 62 associated with each shift register.

Programming

The logic circuits of the integrated circuit 80 operate on a preselectedinternal DC voltage, preferably approximate 9 volts. As shown in blockform in FIG. 2B, the integrated circuit 80 has a voltage regulator 520that accepts an input DC voltage V_(cc), from 12 to 32 volts andproduces the internal DC operating voltage V or V_(ref) of approximately9 volts. The voltage regulator 520 is one of several known types andforms no part of the invention. The signal passing circuitry of theintegrated circuit 80 is activated for passing signals through the datalink module 32 when V_(cc) is within one of two voltage ranges.Preferably the two ranges are approximately 12-15 volts andapproximately 18-32 volts. The DC input voltage V_(cc) and the referencevoltage V_(ref) are fed into a module programmer 232 shown in block formin FIG. 2B. The other input signals fed into the module programmer arePOR, IDATA, ICLOCK, LONG₋₋ RST, ÷2ICLOCK and input signals from theEEPROM 354. The output signals from the module programmer are a programvoltage enable signal (PVE), a program/verify mode signal (P/V₋₋ MODE),a program/verify data signal (P/V₋₋ DATA) and output signals to theEEPROM 354.

The module programmer 232 is shown in more detail in FIG. 12. The moduleprogrammer has a program enabler including a voltage divider 530 havingthree resistors for producing two voltages intermediate V_(cc) andground. One of the two intermediate voltages is fed into a positiveinput 531 of a voltage comparator 532 having as its other input V_(ref).The other of the two intermediate voltages is fed into a negative input533 of another voltage comparator 534 having as its other input V_(ref).When V_(cc) is between approximately 15.5-17.5 volts, the outputs ofeach voltage comparator 532 and 534 is high and a high PVE signal isproduced at the output of an AND gate 536. The module programmer alsoincludes four flip-flops 538, 540, 542 and 544, a four bit counter 546,a five bit counter 548, a serial-to-parallel converter 550, a parallelto serial converter 552, a state machine 553 and at least eleven logicgates 554-565 described in more detail hereinafter. The four bit counter546 is enabled by a low LONG₋₋ RST (which indicates no clock signaltransitions) and it counts 15 transitions of the data line while V_(cc)is between 15.5 and 17.5 volts. The four bit counter 546 is cleared byany transition of the clock signal detected by a delay element 566 andan exclusive-OR gate 555. The four bit output of the counter 546 isAND'd together by gate 556 to set a flip-flop 538. The flip-flop 538 isnormally in a reset state except when the integrated circuit 80 is to beprogrammed or the programming is to be verified in which case theflip-flop is set by the output of the four-bit counter 546. The normallyhigh LONG₋₋ RST is fed into a delay element 570 and a gate 557 whoseoutput is momentarily low on any negative-going transition of LONG₋₋ RSTand the output is fed into a three-input AND gate 558. The other inputsto the three-input AND gate 558 are PVE (a high PVE is an indicationthat the integrated circuit 80 is ready for programming) and an end ofbusy (EOB) signal. The LONG₋₋ RST signal is required to be high prior toprogramming. However, the momentary low on the LONG₋₋ RST input willcause the flip-flop 538 to reset thereby preventing programming. Whenthe three inputs to the AND gate 558 are high, the reset signal to theflip-flop 538 is removed and one of the preliminary steps prior toprogramming is completed. Receipt of fifteen pulses on the data line 46is also required to cause the integrated circuit 80 to become ready forprogramming. Upon receipt of the fifteen pulses, the four bit counter546 produces an output to set the flip-flop 538. The Q output of theflip-flop 538 is designated a PV₋₋ MODE signal. As shown on the timingdiagram, FIG. 18A, the P/V₋₋ MODE signal from FF1 538 transitions fromlow to high at this point 539. A high PV₋₋ MODE is indicative ofprogramming or verifying in progress. A low output Q of the flip-flop538 clears a second flip-flop 540.

The programming data is shifted into the data link module integratedcircuit 80 of the data link module 32 by using the clock line 44 anddata bus 46. As shown in the timing diagram of FIGS. 18A-18C, the datastream contains a program/verify (P/V) bit and 32 data bits. The firstbit is the P/V bit. The P/V bit is low for programming; the P/V bit ishigh for verifying. Flip-flop 540 is enabled by ICLOCK and is set whenthe first programming bit is high. The first 16 bits contain theaddresses for channel A and B (eight bits each). The next 16 bitscontain the control functions including output A to C polarity, output Bto C polarity, mode selection, channel A repeat once, channel A repeattwice, channel B repeat once, channel B repeat twice, and input A/B highselect. The positive true data is placed on the data line at thebeginning of the clock cycle (the positive going edge) and istransferred to the integrated circuit at 180° (negative going edge) ofthe clock cycle. If the P/V bit is high, the next 32 clock cycles willshift the programmed data to the data line. A third flip-flop 542generates a COUNT₋₋ OF₋₋ ONE signal which enables the five-bit counter548 to count to 32 which is the number of programming bits. The thirdflip-flop 542 also enables a serial-to-parallel converter 550 whichreceives serial data from the data bus during programming, that is, whenR/W is low and SH₋₋ EN is low. Data from EE cells (not shown) of theEEPROM 354 is returned to a parallel-to-serial converter 552 forextraction from the integrated circuit 80 via an AND gate 559 and anoutput OR gate 560 during verification of programming. When the Q outputof the second flip-flop 540 is low, an AND gate 561 is enabled and theEE cells are written to. When the Q output of the second flip-flop 540is high, AND gate 559 is enabled and the EE cells are read. The outputof the AND gate 559 is fed into OR gate 560 for outputting data to thedata drive circuit 420 shown in FIG. 14. A five-bit counter 548 counts32 clock cycles thereby enabling AND gate 565 which, in turn, enablesgate 561. When the AND gate 561 is enabled, it provides timing to afourth flip-flop 544 for use in a state machine 553. The output of thefourth flip-flop 544 provides a program ready signal (PROG₋₋ RDY) to thestate machine 553. As shown in the timing diagram, FIG. 18C, the PROG₋₋RDY signal goes high during the electrically erasable program cycle 574.The state machine 553 controls the burning in of the EE cells through aprogramming control logic circuit 572. The state machine 553 has anoutput OBUSY that clears flip-flop 544 through gates 562 and 563 anddelay element 568. Each data bit will be placed on the data line 46 atthe beginning of each clock cycle where it can be read by a programmer.This data is negative true.

The programming supply voltage at the integrated circuit 80 must be 16.5Vdc±1.0 Vdc. Preferably, a buffer resister of 300Ω (not shown) is usedbetween the bus and the integrated circuit 80. The voltage loss acrossthis resistor is approximately 0.4 Vdc. The supply voltage compensatesfor this loss.

The voltage levels for the two control lines (clock line 44 and dataline 46) swings between bus common and the internal operating voltage Vof the integrated circuit 80. This voltage is preferably nine volts. A100 k Ω resistor (not shown) is part of each input filter 188 and 186for the clock lines 44 and data line 46, respectively. These resistorsare also used for input protection for the integrated circuit 80. Thiswill allow an input signal of several hundred volts without damaging theintegrated circuit 80 or causing improper operation. To make theprogrammer clock and data signals compatible with the prior art datalink module described in the aforesaid patent of Riley, a 12 volt signalis used. During programming, the clock frequency is between 25 kHz and30 kHz. The clock frequency is used for timing reference in writing tothe EEPROM 354.

As shown in the timing diagram for the program cycle, FIG. 18C, afterthe data has been shifted into the integrated circuit 80, theelectrically erasable programming cycle 574 requires 2 cycles of 200 mseach. The first cycle is an erase cycle, the second cycle is theprogramming of the EEPROM 354. The output of the four-input AND gate 564is fed into the enabling clock input of the serial-to-parallel converter550. The programming time for each integrated circuit 80 isapproximately 500 ms.

The method of programming the data link module over the data bus line 46and the master clock line 44 includes: Step one, removing the power fromthe data link module 32. Step two, applying 15.5 to 17.5 volts directcurrent to the V_(cc) terminal of the data link module 32. Step three,waiting a preselected time for the power on reset circuit 190 to producea low POR signal. Step four, holding the CLOCK signal high forpreselected interval of time, 70 FIG. 18A, preferably for at east 5μsec, and then, while continuing to hold the CLOCK signal high,simultaneously transitioning the data line fifteen times between highand low. Step five, applying the master clock signal 85 to the masterclock terminal 84 of the data link module 32 and waiting for apreselected number of cycles of OSC in order for LONG₋₋ RESET to gohigh. Step six, sending a logical high P/V bit 578, FIG. 18B, over thedata line 46. Step seven, sending 32 bits of programming data over thedata line 46. Step eight, waiting a preselected time for the statemachine 553 and the programming control logic circuit 572 to "burn" theprogrammed bits into the respective cells of the EEPROM 354. The detailsof the state machine 553 and programming control logic circuit 572 arewell known in the art and form no part of the invention. Step nine,applying direct current to the V_(cc) terminal at a voltage above 17.5volts or below 15.5 volts. Step ten, operating the data link module 32in accordance with the programming. Between step six and step seven theperson programming the module programmer 232 has the advantageous optionof pausing an indefinite time interval to prepare for the actualprogramming. This time interval is shown in the timing diagram, FIGS.18A and 18B as interval 576. The actual programming is preferablyaccomplished by means of a hand-held programming device, the details ofwhich are well known and form no part of the invention. Theaforementioned steps assure that the data link module is notinadvertently programmed by noise on the data line 46.

As shown on the timing diagram, FIGS. 18D-18F, the method of verifyingthe programming of the data link module is similar to the method ofprogramming except as follows. Step six, sending a logical low P/V bit596, FIG. 18E, over the data line 46. Step seven, receiving 32 bits ofprogramming over the data line 46 via the P/V₋₋ DATA line and the DATA₋₋DRIVE output.

Input/Output Word Extenders

Unlike known data link modules, the data link 32 selectively passeseither single bits of data or multi-bit words of data, preferably 8-bitto 16-bit words of data, from an input device 50 to the data bus 46 orfrom the bus to an output device 54. As shown in FIG. 15, the data linkmodule 32 has an integrated circuit 80 and preferably two shiftregisters 588 and 590. Shift register 588 is preferably a 16-bitparallel-to-serial shift register for parallel receipt of data from a16-bit input field device 580 and serial transmission of this data ontothe data bus 46. Shift register 590 is preferably a serial-to-parallel16-bit shift register for parallel transfer of data to a 16-bit outputfield device 582. The data link module 32 shown in FIG. 15 is usedeither as an input module, as an output module, or, when operated inmode two, concurrently as both an input and output module.

Unlike prior data link module integrated circuits, the data link module32 includes a shift clock in terminal 104 having on it a reproduction ofthe master clock signal 85 during a time interval 453, FIGS. 17A-17D,between an address A time slot 422 and an address B time slot 424. Inaddition, the integrated circuit 80 has a shift clock out 106 terminalhaving on it, in mode one, an inverted master clock signal 85 during thetime interval 453 between the address A time slot 422 and the address Btime slot 424. A word extender circuit 430, FIG. 2A, produces a SHIFT₋₋CLK₋₋ IN signal and a SHIFT₋₋ CLK₋₋ OUT signal at output terminals 104and 106, respectively, of the integrated circuit 80.

The word extender 430, shown in more detail in FIG. 13, has six inputterminals 431-436 and two output terminals 437 and 438. A mode controlcircuit 440 generates a MODE₋₋ CLK signal which is the ICLOCK signalwhen EE cell 441 is low and is a ÷2ICLOCK signal when the EE cell 441 ishigh. The word extender circuit 430 includes a data flip-flop 450 whichis cleared by SYNC at the beginning of each frame 62. The flip-flop 450normally has a low output 451 due to the grounded input 452. The COIN₋₋A signal presets the flip-flop and makes output 451 high. The output 451remains high until the occurrence of the high COIN₋₋ B signal.

The output 451 of the flip-flop 450 is fed into an AND gate 455 alongwith MODE₋₋ CLK. The output 444 of the AND gate 455 produces SHIFT₋₋CLK₋₋ OUT and it is a reproduction of ÷2ICLOCK during mode twooperation.

The word extender circuit 430 also has a three-input 445-447 AND gate460. An OUTPUT₋₋ WINDOW signal 121 is fed into input 447 of the AND gate460. The OUTPUT₋₋ WINDOW signal is produced by the window controlcircuit 120 shown in block form in FIG. 2A. FIG. 13 also shows, indetail, a portion of the window control circuit 120 which generates theOUTPUT₋₋ WINDOW signal. The OUTPUT₋₋ WINDOW signal is always low in modeone operation, thereby continuously enabling AND gate 460. During modetwo operation, OUTPUT₋₋ WINDOW is low only during the second half ofeach ÷2ICLOCK cycle, thereby enabling AND gate 460 only during thesecond half of each ÷2ICLOCK cycle. The ICLOCK signal is fed into input458 of AND gate 460, thereby enabling the AND gate only during thesecond half of each ICLOCK cycle. Therefore, during mode two, output 461of AND gate 460 is a train of positive and negative pulses of dissimilarwidths commencing at time slot A 422' and ending at time slot B 424'.The output 461 of AND gate 460 produces the SHIFT₋₋ CLK₋₋ IN signal. TheSHIFT₋₋ CLK₋₋ IN signal produced during mode two operation is shown inFIGS. 17C and 17D.

Referring again to FIG. 15, output signals from a controlling fielddevice 580 are fed into input terminals of a synchronousparallel-to-serial shift register 588. The mode/sync output terminal 110of the integrated circuit 80 is connected to the shift high/load (SH/LD)terminal 584 of the parallel-to-serial shift register 588. The syncpulse 196 at the mode/sync output terminal 110 controls the loading ofthe shift register 588. A shift clock out (SCO) terminal 106 of theintegrated circuit 80 is connected to the clocking terminal 585 of theparallel-to-serial shift register 588. The DATA₋₋ OUT terminal 586 ofthe parallel-to-serial shift register 588 is connected to-the data bus46 via an OR gate 598 and an FET 600. Data in the parallel-to-serialshift register 588 is transferred to the data bus 46 on the falling edgeof each SCO pulse. The falling edge of each SCO pulse occurs at thebeginning of each time slot 65.

The data bus 46 is connected to the DATA₋₋ IN terminal 592 of asynchronous serial-to-parallel shift register 590. The synchronousserial-to-parallel shift register 590 is clocked by the shift clock in(SCI) signal inputted at 594 for copying data from the data bus 46 onthe falling edge of each SCI pulse. The falling edge of each SCI pulseoccurs one half of a master clock cycle 85 after the occurrence of thefalling edge of each SCO pulse. The output signals of theserial-to-parallel shift register 590 are fed into input terminals of afield device 582 controlled by the data link module 32.

In mode two, data is copied from the parallel-to-serial shift register588 to the data bus 46 during the first half of the master clock cycle85 and data is copied from the bus to the serial-to-parallel shiftregister 590 during the second half of the clock cycle. As shown on thetiming diagram in FIGS. 17C and 17D, in mode two, the SCO signal is aninverted master clock signal 85 and the SCI signal is a pulse train ofdissimilar width positive and negative pulses. The dissimilar widthpulses are due to OUTPUT₋₋ WINDOW being fed into the SCI AND gate 460,FIG. 13. The purpose of the dissimilar width SCI pulses is to ensurethat the falling edge of each SCI pulse occurs within the second half ofthe mode two time slot 67.

High Voltage Protection Circuit

Unlike known data link modules, the integrated circuit 80 of the datalink module isolates the channel A and channel B input signals,respectively, from the data bus 46. Unlike the prior art, the signalsfed into the input terminals 94 and 96 of the integrated circuit 80 donot control the bus voltage by driving an internal transistor. Rather,the integrated circuit 80 includes a data drive output terminal 112 forconnection to an external transistor 600, FIG. 14, for bringing the busvoltage low. As shown in block form in FIG. 2A, a data drive circuit 420produces a DATA₋₋ DRIVE signal. As shown in FIG. 14, the data linkmodule 32 includes an integrated circuit 80 and transistor 600 externalto the integrated circuit. A resistor 602 is placed at the data businput terminal 86, preferably 100 kΩ, in order to limit current into theintegrated circuit 80. The data drive circuit 420 is comprised of athree-input OR gate 604 having as inputs P/V₋₋ DATA, INPUT₋₋ DATA₋₋ Aand INPUT₋₋ DATA₋₋ B. The signal at the output terminal 606 of the ORgate 604 passes through a resistor 608 and is available outside of theintegrated circuit 80 as the DATA₋₋ DRIVE signal 610 on a data driveterminal 112. The DATA₋₋ DRIVE signal 610 drives the external transistor600. FIG. 14 shows a field effect transistor 600 (FET); however, abipolar transistor is alternatively used. The data bus drive output 610is connected to the gate 612 of the FET 600. The source 614 of the FET600 is connected to the data bus 46 through a resistor 616, preferably a10Ω resistor. The drain 618 of the FET 600 is connected to ground 48.The transistor 600 is not used as an amplifier but rather as a switch.

During the time slot 422 associated with channel A, INPUT₋₋ DATA₋₋ A ishigh. The high INPUT₋₋ DATA₋₋ A causes the data bus drive signal to behigh, thereby causing the FET 600 to conduct. When the FET 600 conductsthe data bus voltage is reduced from approximately 9.0 volts which is alogical low or a logical zero to approximately 0.7 volts which is alogical high or a logical one. In a similar fashion, a high INPUT₋₋DATA₋₋ B and a high P/V₋₋ DATA cause the FET 600 to conduct.

The external transistor 600 is advantageous because it allows theintegrated circuit 80 of the data link module 32 to accept higher databus voltages and currents than the transistor internal to the prior artintegrated circuit can accept without breaking down. Furthermore, whenexposed to an very high bus voltage, the external transistor 600 breaksdown and becomes a low impedance source and protects the relativelyexpensive integrated circuit 80 from damage. The external transistor 600advantageously breaks down when the bus voltage reaches about 60 voltsthereby protecting the integrated circuit 80 which can withstand atleast 60 volts on the bus 46. The external transistor 600 can be easilyand inexpensively replaced if damaged; whereas, the integrated circuit80 is relatively expensive and more difficult to replace.

Data Bus Integrity Checker

Referring now to FIGS. 15 and 16, the data link module 32 has a data busintegrity checker 630 for determining the integrity of the data bus 46.When the integrity checker 630 determines a presence of a fault, thechecker 630 prevents the integrated circuit 80 of the data link module32 from receiving data signals 87 and clock signals 85 from the data bus46 and clock bus 44, respectively. The checker 630 will respond to threetypes of faults: a fault to ground; a fault to the operating voltage,including a fault to some voltage intermediate the operating voltage andground; and a floating or open bus fault. The checker 630 is located onmodules 32 that act as output modules 56, such as the data link module32 shown in FIG. 16, and on modules that act as both input modules 52and output modules 56, such as the data link module 32 shown in FIG. 15.However, the checker 630 is alternatively present on all modules 32, butdoes not function on input modules 52.

In the preferred embodiment, the checker 630 is external to theintegrated circuit 80 and mounted on the data link module 32. Referringnow to FIG. 16, the checker 630 is comprised of three input terminals: aterminal 632 for receipt of signals 87 from the data bus 46, a terminal634 for receipt of signals 85 from the clock bus 44, and a terminal 636for receipt of the mode/sync signal 456 from the integrated circuit 80.The checker 630 has two output terminals: a terminal 638 for connectionto the data terminal 86 of the integrated circuit 80, and a terminal 640for connection to the clock terminal 84 of the integrated circuit 80.

The checker 630 acts during the sync period 58 between each time frame62, FIG. 21. The sync period 58 is produced by the master clock module36 when the master clock module periodically stops placing clockingpulses on the clock bus 44. A properly functioning data bus 46 is at arelatively high positive voltage (preferably 9-12 volts) when no datasignals 87 are present on the data bus. The checker 630 works inconjunction with the master clock module 36. The master clock module 36exercises the data bus 46 during each sync period 58 by bringing thedata bus low for an interval 648, FIG. 20A, and then allowing the databus to return to its normally high positive voltage state. When themaster clock module 36 brings the data bus 46 low it simulates thepresence of a signal 87', FIG. 20A, on the data bus. The duration of theinterval 648 is not critical; however, the interval 648 is long enoughfor all logic elements to stabilize but not longer than half the syncperiod 58. The circuit at the master clock module 36 which brings thedata bus low is well known and forms no part of the invention. In modetwo operation, the computer interface card (not shown), instead of theclock module 36, performs the operation of bringing the data bus 46 low.

The data bus checker 630 looks for the simulated signal 87' during thesync period 58 between each time frame 62. The checker 630 allows normaloperation if the simulated data bus signal 87' is detected. However, thechecker 630 prevents the one data link module 32 on which it is mountedfrom receiving data signals 87 intended for that one module if thesimulated signal 87' is not detected. The checker 630 also has anindicator 642 to alert the operator of the control system 30 of thecondition of a data bus line 633 leading to the data link module 32 onwhich the checker is mounted. In this context, the data bus line 633,FIG. 1, is a branch of the data bus 46.

The circuit 631 of the data bus integrity checker 630 is shown in detailin FIG. 19. A two-input NAND gate 644 has both inputs 646 and 647connected to the data bus 46; however, a delay element with a logicalinverter 652 is between the data bus 46 and one 647 of the two inputs ofthe NAND gate. The output terminal 654 of the NAND gate 644 is coupledto the S input terminal 656 of an RS flip-flop 658. When the data busvoltage transitions from low to high, a relatively short duration pulsefrom the output of the NAND gate 644 sets the RS flip-flop 658. In asimilar manner, another NAND gate 660 and another delay element/inverter662 produces another relatively short duration pulse at everylow-to-high transition of the master clock signal 85. An output terminal666 of the other NAND gate 660 is coupled to the R input terminal 668 ofthe RS flip-flop 658. The RS flip-flop 658 is reset on every clock cycle61; however, during the sync period 58 there are no clock cycles, bydefinition. The two NAND gates 644 and 660 and the two delay elements650 and 662 are Schmitt triggered in order to perform in ahysteresis-like manner to overcome the slow rise time of the data 87 andclock 85 signals and to overcome the noise on the data 46 and clock 44buses.

A synchronous data flip-flop 670 has its D input terminal 672 connectedto the power supply positive voltage source (not shown), thereby causingthe flip-flop 670 to set whenever the clocking input terminal 674 to theflip-flop 670 goes high. The clocking input terminal 674 isedge-sensitive and it responds only to a rising edge of a signal. A line676 is connected from the delay element/inverter 662 to a clocking input674 of the flip-flop 670. Therefore, the flip-flop 670 is set on everyhigh-to-low transition of the master clock signal 85. When the flip-flop670 is set, a signal XSYNC 664, FIGS. 20A and 20B, appears on a Q outputterminal 665 of the flip-flop 670.

At start up, the flip-flop 670 is initially asynchronously set by alocal power-on-reset circuit 696 comprising a diode 678, a resistor 680and a capacitor 682 connected to the preset terminal (PRE) 683 of theflip-flop. The integrated circuit 80 produces a sync pulse 196 at itsmode/sync terminal 110 during the sync period 58. An AND gate 684 incombination with a delay element/inverter 686 produces a reset pulse688, FIGS. 20A and 20B, at the output 685 of the AND gate on alow-to-high transition of the sync pulse 196. The reset pulse 688 is fedinto a asynchronous clear terminal (CLR) 689 of the flip-flop 670.Therefore, the signal XSYNC produced at the output Q 674 of theflip-flop 670 goes low in response to the sync pulse 196 on themode/sync terminal 110 of the integrated circuit 80. On the nextlow-to-high transition of the signal at the clocking terminal 674 of theflip-flop 670 (which occurs on the high-to-low transition of the masterclock signal 85), the flip-flop is set again and it remains set untilthe next time frame 62.

A second data flip-flop 690 has its D input terminal 692 connected tothe Q output terminal 694 of the RS flip-flop 658. The second dataflip-flop 690 has its clocking terminal 696 connected to the Q outputterminal 674 of the data flip-flop 670. The asynchronous clear terminal(CLR) 698 of the second data flip-flop 690 is connected to the localpower-on-reset circuit 676. On start-up, the flip-flop 690 is reset andits Q output terminal 702 is low. As a result, at start-up, a lightemitting diode (LED) 704 connected between the power supply positivevoltage source and the Q terminal 702 illuminates. In addition, atstart-up, aQ output terminal 706 goes high, thereby placing the powersupply positive voltage on the flip-flop 690 side of two diodes 708 and710.

The second data flip-flop 690 samples the state of the Q output terminal694 of the RS flip-flop 658 at the end of each sync period 58. If arising edge is detected on the data bus 46 by the first data flip-flop670 during the sync period 58, the Q output terminal 702 of the seconddata flip-flop 690 latches high, reverse biasing the diodes 708 and 710,and allowing normal operation of the data link module 32. The LED 704remains off. However, if a rising edge is not detected, the Q outputterminal 702 of the second data flip-flop 690 is latched low. Theabsence of a rising edge is indicative of a fault on the data bus 46.The low Q output causes the LED 704 to illuminate. Diode 708 isconnected between the Q terminal 706 of the flip-flop 690 and the databus input terminal 86 of the integrated circuit 80. Diode 710 isconnected between the Q terminal 706 of the flip-flop 690 and the clockbus input terminal 84 of the integrated circuit 80. If the data bus 46has a fault, the Q terminal 706 of the flip-flop 690 will be high. Ahigh Q output terminal 706 cause the two diodes 708 and 710 to conductwhen the clock bus 44 or data bus 46 lines attempt to go low. When diode708 is conducting, the data bus input terminal 86 of the integratedcircuit 80 remains at the high voltage level indicative of an absence ofa data signal 87. The integrated circuit 80 is thereby immediatelyprevented from responding to signals 87 on the data bus 46. Unlike theprior art, there is no significant delay between the detection of afault and the prevention of the data link module 32 from receiving databus signals 87. Therefore, unlike the prior art, the data link module 32will immediately stop producing control signals for controlling one ormore output devices connected to the data link module 32. This isadvantageous because any signals 87 on the data bus 46 are probablyfalse due to the fault condition on the data bus. When diode 710 isconducting, the clock bus input terminal 84 of the integrated circuit 80remains at the high voltage level thereby producing, at only the onedata link module 32, a loss of the master clock signal 85. The clockloss detect circuit 240 internal to the integrated circuit 80 turns offthe integrated circuit shortly after the checker 630 began preventingthe integrated circuit from receiving the master clock signal 85.

While a detailed description of the preferred embodiment of theinvention has been given, it should be appreciated that many variationscan be made thereto without departing from the scope of the invention asset forth in the appended claims.

I claim:
 1. An I/O module coupled to a data bus for use in a timedivision multiplexing control system, the I/O module including an inputcircuit for generating a data output signal to the data bus in responseto input signals from an input device coupled to the I/O module, theinput signal having an input voltage level range based on the inputdevice having one of a plurality of voltage ratings, the I/O modulecomprising:A. an input terminal for receiving the input signal from theinput device; B. a memory for storing one of a plurality of differentinput voltage levels acceptable to the I/O module; C. a programmableinput voltage level selector to select an input device voltage levelrange from the stored plurality of different input voltage levels; D. acontrol circuit responsive to the stored input voltage level; and E. alogic circuit for producing the data output signal in combination withthe stored one of a plurality of different input voltage levels and thereceived input signal being within the selected input device voltagelevel range.
 2. The I/O module of claim 1 wherein the memory furtherincludes means for changing the selected input device voltage level. 3.The I/O module of claim 2 wherein the memory for storing the inputvoltage level is an electronically erasable programmable read onlymemory (EEPROM).
 4. The I/O module of claim 3 further including a signalconditioning circuit coupled between the input terminal and the controlcircuit and having an anti-aliasing filter.
 5. The I/O module of claim 3wherein the anti-aliasing filter has a bandstop of 30 kHz, allowinginput signals of 3 kHz to pass through the filter and to the controlcircuit.
 6. The I/O module of claim 5 wherein the control circuitcomprises a hysteresis circuit having a first input for receiving thepassed through input signal from the anti-aliasing filter and a secondinput for receiving an output from the memory representative of thestored input voltage level, the hysteresis circuit for preventing falsestate transitions of the input signal due to voltage level variations.7. The I/O module of claim 6 wherein the hysteresis circuit is a 50%hysteresis circuit.
 8. The I/O module of claim 7 wherein the hysteresiscircuit includes a first comparator for setting a minimum voltage levelthreshold representative of the input device being in one logic stateand a second comparator for setting a maximum voltage level thresholdrepresentative of the input device being in another logic state.
 9. TheI/O module of claim 8 wherein the minimum voltage level thresholdrepresentative of the input device being in one logic state is 75% ofthe stored input voltage level.
 10. The I/O module of claim 7 whereinthe maximum voltage level threshold representative of the input devicebeing in another logic state is 25% of the stored input voltage level.